Control system for preventing the skidding of vehicles

ABSTRACT

Equipment for of preventing the skidding of vehicles by detecting the speed of wheel rotation, comparing the detected speed with a set reference value of the deceleration characteristics, controlling the wheel rotation by suitably effecting three kinds of braking pressure operations, namely decreasing, maintaining and increasing the braking pressure, in response to the results of the aforesaid comparison, and renewing the reference speed value to be newly set when the wheel speed exceeds the previous set reference value by a predetermined amount, and an apparatus for modulating the wheel braking pressure at a required value adapted to driving conditions.

1 States atent s41 CONTROL SYSTEM FOR PREVENTING 3,441,320 4/1969 Flory..3o3/21 B THE SKIDDING OF VEHICLES 3,547,501 12/1970 3,482,887 12.1969[72] Inventor: Takes!"OchmTWmaJaPa" 3,498,682 3i1970 Mueller et al...303/21 BE 3,582,152 6/1971 Burckhardt et a1. ..303/21BEUX [73]Assignee: Toyota Jidosha Kabushiki Kaisha,

Toyota-shi, Japan [22] Filed: April 3, 1970 [21] Appl. No.: 25,436

[30] Foreign Application Priority Data Oct. 20, 1969 Japan ..44/s375sOct. 20, 1969 Japan ..44/99575 [52] U.S.Cl ..303/21BE,188/181C, 303/21F[51] Int. Cl. ..B6od 8/08 [58] Field of Search ...188/l8l; 303/20, 21;307/120, 307/121; 317/5; 324/160, 161, 162; 340/262,

Primary Examiner-George E. A. l-lalvosa Assistant Examiner-Stephen G.Kunin Attorney-Cushman, Darby & Cushman [5 7 ABSTRACT Equipment for ofpreventing the skidding of vehicles by detecting the speed of wheelrotation, comparing the detected speed with a set reference value of thedeceleration characteristics, controlling the wheel rotation by suitablyeffecting three kinds of braking pressure operations, namely decreasing,maintaining and increasing the braking pressure, in response to theresults of the aforesaid comparison, and renewing the 263 referencespeed value to be newly set when the wheel speed exceeds the previousset reference value by a [5 6] References C'ted predetermined amount,and an apparatus for modulat- UNITED STATES PATENTS ing the wheelbraking pressure at a required value adapted to driving conditions.3,245,213 4/1966 Thompson et a1. ..303/21 EB 3,467,444 9/1969 Leiber..188/181 A X 6 Claims, 11 Drawing Figures COMPARATOR o E 7L7 c3 /2 cMPARATOLQJKINVERTERS /3 AMPLI FIES /4 PATENTEDBEB 12 I 12 3,705. 748

SHEET 1 OF 4 F/GI/ COMPARATOR COMPARATOR C MPAR MJQNVERTERS I INVENTORSATTORNEYL PATENTEU nan 12 I972 SHEET 0F 4 FIG 5 T/ME L HPE$$UR DECREASEPRESSZWE INCREASE L2 HPESSUPE RE7ENT/0N PRESSURE RELEASE p HmfiAUL/CPRESSURE INVENTORS TMESH/ 10/ BY @Mwm v y ATTORNEYS CONTROL SYSTEM FORPREVENTING THE SKIDDING OF VEHICLES BACKGROUND OF THE INVENTION 1. Fieldof the Invention This invention relates to a method of preventing theskidding of vehicles to prevent a running vehicle from loosing adirectional control and the spinning of the vehicle body due to thelocking of the vehicle wheels caused by applying a braking force to arunning vehicle, and it also refers to a wheel braking pressuremodulator for modulating a hydraulic wheel braking pressure at arequired value adapted to driving conditions of the vehicle body whichchange from time to time.

2. Description of the Prior Art Anti-skid methods heretofor contemplatedand practiced include the one in which a braking force is applied andreleased on the basis of the detection of deceleration or accelerationof the vehicle wheels to prevent locking thereof, and the one, in whichthe speed of wheel rotation is controlled on the basis of the results ofcomputation of the actual vehicle speed carried out by utilizing adeceleration detector and the like. By the former method, changes in thespeed of wheel rotation are large and sudden, so that passengers in thecar are made to feel very uncomfortable. The latter method involves adeceleration detector possessing a complicated construction and of highcost and may result in occassional errors due to the slopes of roads andthe turning posture of the vehicle itself.

SUMMARY OF THE INVENTION An object of the invention is to provideequipment for preventing the skidding of vehicles by means of anarrangement including a rotational speed detector, a charging anddischarging circuit consisting of a diode, a capacitor, resistors, threecomparators respectively connected to associated logic circuits and twocoils for the ON-OFF operation of the respective electromagnetic valvesof a hydraulic braking pressure modulator, which effects three kinds ofbraking pressure operations by making the hydraulic braking pressurerise or fall substantially linearly when the braking pressure isincreased or decreased and by maintaining the braking pressure of avalue of that time constant during the maintaining operation throughswitching over or closing air passages so as to control the speed of therotation of the wheels.

Another object of the invention is to provide a wheel braking pressuremodulator, which is capable of selectively supplying an externalelectric control pattern of signals which are most suitable for thebraking operation by the above method according to the present inventionand is possible to manufacture at a comparatively low cost.

One of the features of the invention is the possibility of controllingthe braking pressure when an vehicle travels along a road having anyvalue of coefficient of friction whereby the increasing process of thespeed of rotation of the vehicle wheels in continuously detected by thedetecting circuit detecting the charging of the capacitor andmaintaining the value of the hydraulic pressure at that time until thevehicle wheel speed goes up near the actual speed of the vehicle,whereupon the renewal of a reference voltage is automatically carriedout.

The second feature of the invention is that control can be accomplishedeffectively, even if the coefficient of friction of the road surfacechanges during braking operation, because the reference voltage is thenautomatically renewed.

The third feature of the invention is the incorporation of the functionof maintaining the hydraulic pressure, whereby the change of thehydraulic pressure may be reduced as compared with the conventionalantiskid apparatus provided with no maintaining operation to reduceshock experienced by the vehicle body at the time of braking operations(which is a great advantage).

The fourth feature of the invention is that the detecting circuit issimple in construction and very inexpensive in manufacturing cost, asthe negative pressure in an intake manifold is utilized as the drivingsource and only the speed of the wheel rotation is detected, which is ofgreat practical value in preventing the skidding of vehicle.

The fifth feature of the invention is the selective availability of anexternal electrically controlled pattern of signals most suited to theanti-skid braking of the wheels with a hydraulic pressure modulatoraccording to the invention. Therefore, the braking pressure may befreely increased and decreased. Also, the braking pressure may bemaintained at a value and may be released, which can be appropriatelycombined with the above functions of increasing and decreasing thebraking pressure. Further, the rate of increase or decrease of thebraking pressure may be controlled. Thus, an excellent anti-skid brakingperformance suitable for changes in various complicated travellingconditions of the vehicle may be obtained, which cannot be achieved withthe conventional apparatus and therefore brings a great practical meritin addition to a simple construction and inexpensive manufacturingcosts.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a circuit diagram showingthe principle of a preferred embodiment of the invention;

FIG. 2 is a sectional view of an embodiment of the apparatus forcontrolling the hydraulic braking pressure in accordance with theinvention;

FIGS. 3a to 3g illustrate operational functions of various parts of theembodiments shown in FIGS. 1 and 2;

FIG. 4 is a circuit diagram showing the principle of a furtherembodiment of the invention; and

FIG. 5 illustrates the operational functions of various parts of theembodiment shown in FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention is now describedin conjunction with preferred embodiments thereof with reference to theaccompanying drawings. Referring to FIG. 1, which illustrates theprinciples of operation of one embodiment of the present invention,numeral 1 designates a wheel rotation speed detector. It may be a DCgenerator or a combination of a pulse generator which sends out pulsesresponding to the revolution and a frequency-tovoltage converter, eitherof the generators being coupled to a wheel or a propeller shaft. SymbolV designates the DC output voltage which is proportional to the speed ofwheel rotation. Numeral 2 designates a diode. It is assumed to be anideal diode having zero forward resistance and an infinite reverseresistance. Numerals 3 and designate resistors respectively, and numeral5 designates a capacitor with a capacitance value C. The resistor 3serves to detect the charging and discharging of the capacitor and has aresistance value R,, which is made sufficiently low as compared with theresistance value R of the resistor 4 so that the time constant T, whichequals R,C may have a value (T, 0.001 0.0001 sec) so that it cansufficiently follow the changes in the speed of wheel rotation. Theresistor 4 determines the discharge characteristic of the capacitor,with the time constant of T (R, R C z R C being so set as to have avoltage drop charac teristic with voltage values corresponding to theamount equal to or higher than the actual deceleration of the vehicle,i.e., the deceleration under the optimum braking conditions when theroad surface is the least slippery one.

A bias voltage E is applied to increase the potential difference acrossthe capacitor 5 so that the rate of change of the potential differenceacross the capacitor attendant on the variation in the voltageproportional to the wheel speed is reduced to make the dischargingcurrent flowing through the resistors 3 and 4 substantially constant,thereby making the voltage drop characteristic across the capacitor 5substantially linear. In other words, the application of the biasvoltage E causes the resistors 3 and 4 to constitute a constant currentdischarging circuit. Numeral 6 designates an ideal impedancetransforming operational amplifier having an infinite input impedance, azero output impedance and a unity amplification degree. The outputvoltage is taken as the first reference voltage V,, and it is divided byresistors 7 and 8 to produce the second reference voltage o'V,, where a'should be determined to match the characteristics of the vehicle and thecontrol system so as to obtain a minimum braking distance (a ispreferably 0.95 to 1.00). Numeral 9 designates a comparator with anoutput C, being at a level 1 when V,, is equal to V, and at zero levelwhen V,,, is lower than V Numeral 10 designates another comparator withan output C being at level 1 when V is higher than or equal to UV, andat zero level when V,, is lower than o'V,. Numeral 111 designates adifferential comparator with an output C being at level l when thepotential at the end of the resistor 3 on the side of the diode 2 ishigher than the potential at the end of the resistor 3 on the side ofthe capacitor 5, that is, when the capacitor 5 is being charged, andwith the output (I, being at zero level otherwise. Numerals l2 and 13designate inverter circuits, 14 an AND circuit, 15 an OR circuit, 16 and17 solenoid valve drive amplifiers, 18 a solenoid with a coil L, forincreasing and decreasing the hydraulic pressure, and 119 a solenoidwith a coil L, for maintaining the hydraulic pressure.

With respect to the operation of the logical circuits and the amplifiersdesignated as 12 through 17, when C, is at zero level (V,, 0V the coilL, is made ON through the inverter circuit 12 and the solenoid valvedrive amplifier 16, and otherwise it is OFF. The coil L, is made ONthrough the circuit consisting of the inverter 13, the AND circuit 14,the OR circuit 115 and the solenoid valve drive amplifier l7 when C, isat zero level (V,,, V and C is at level 1 (V,,, UV,), or also it is madeON through the circuit consisting of the 0R circuit and the solenoidvalve drive amplifier 17 when C, is at level 1 (when the capacitor 5 isbeing charged), and otherwise the coil L, is OFF.

FIG. 2 shows the construction of a hydraulic pressure modulator as oneof the embodiments of the wheel braking pressure modulator employed inthis invention. It comprises a body 51, a diaphragm 52 urged by atension spring 53 and dividing the inside space intotwo chambers A andB, a pressure increasing or decreasing rod 54 which is fixed to thediaphragm 52, a ball 55 of a stop valve to stop communication betweenthe master cylinder and the wheel cylinder, a tension spring 56 urgingthe ball 55, an air-chamber (A) or 57,- another air-chamber (B) or 58, apressure maintaining electromagnetic valve 59 for maintaining thehydraulic pressure including a return spring 40 and a solenoid 19 with acoil L3 for driving the pressure maintaining electromagnetic valve 59, apressure increasing and decreasing electromagnetic valve 42 including areturn spring 43 and a valve drive solenoid 18 with a coil L,, athrottle valve 45 to control the rate of pressure increase and athrottle valve 46 to control the rate ofpressure decrease. The negativepressure in the intake manifold and the atmospheric pressure are assumedto be constant in the following description regarding the operation ofthe apparatus is shown in FIG. 2. The illustrated apparatus shown in astate for normal driving. The diaphragm 52 is at the rightmost position,and the stop valve is open, communicating the master cylinder with thewheel cylinder. There-is no current flowing through both the solenoidcoil L, and L and both of the electromagnetic valves are at their lowpositions, so that the chamber A is under a negative pressure. Thechamber B is always under the negative pressure through a piping. Wenthe driver depresses the brake pedal, the hydraulic brake pressure inincreased to urge the pressure control rod 54 more leftward, whichmovement is, however, too small to close the stop valve on account ofthe tension spring 53 which presents the leftward displacement of therod 54. Upon starting the pressure control by causing a current to flowthrough the solenoid coil L, the valve 42 is moved to its upperposition, communicating the chamber A with the external atmosphere toadmit air therein so as to increase the air pressure, thereby moving thediaphragm 52 leftward, so tat the top valve is closed to stopcommunication between the master cylinder and the wheel cylinder. As thediaphragm continues to move leftward, the hydraulic pressure on thewheel cylinder side begins to decrease. when a current flows through thesolenoid coil L to move the valve 59 to its upper position, the flow ofair into the chamber A is stopped to cease movement of the diaphragm 52so as to maintain the hydraulic pressure in the wheel cylinder at avalue of the instant that the diaphragm movement ceases. Then, bycutting ofi the current which flows through both the solenoid coils L,L,, the chamber A is switched over to communicate with the negativepressure to decrease the air pressure in the chamber A, so that thespring 53 urges the diaphragm back to the right to increase thehydraulic pressure in the wheel cylinder. If, in this course, a currentflows again through the coil L,, the chamber A is shut off with thenegative pressure therein to stop the diaphragm movement so as tomaintain the hydraulic pressure at a value of that time. In this manner,by means of two solenoid actuated valves, the three functions ofincreasing, decreasing and maintaining the pressure may be attained.Furthermore, it is possible to control the rate of pressure increase anddecrease by regulating the air flow rate by means of the throttle valves45 and 46.

FIGS. 3a to 3g illustrates the functions of various parts in thepreceding embodiment of the invention with the abscissa commonlyrepresenting time and the ordinate of FIG. 3a taken for voltages,wherein V represents the actual speed of the vehicle (which is indicatedby a dashed line, as it is difficult to know surely during the actualbraking operation), V,,, the wheel rotation speed, V, the firstreference voltage, and UV, the second reference voltage. With respect tothe abscissae of the other figures, in FIG.-3b, it represents the outputC from the comparator 9; in FIG. 30, the output C, from the comparator10; in FIG. 3d, the output C, from the comparator 11; in FIG. 3e, thecurrent which flows through the solenoid coil L in FIG. 3f, the currentwhich flows through the selenoid coil L and, in FIG. 3g, the variationin the hydraulic pressure in the wheel cylinder (hereinafter merelyreferred to as the hydraulic pressure).

The functions of the method of the present invention are now describedaccording to the lapse of time as illustrated in FIGS. 3a to 33. Thenormal driving state is maintained until the time t when the driverdepresses the brake pedal, the hydraulic pressure begins to increase andthe wheel rotation speed represented by V,,, becomes lower than theactual vehicle speed represented by V. Until V becomes lower than thefirst reference voltage V,, however, V remains equal to V,. Thus, C, isat level 1, C is at level 1, and C is at zero level. Therefore, thereare no currents flowing through the coils L, and L so that the state ofthe hydraulic pressure modulator remains unchanged. The hydraulicpressure, however, continues to increase when the brake pedal iscontinuously depressed. At the time t V,, becomes lower than V,, so thatthe level of C, is shifted to zero level, while C remains'at level 1 andC, at zero level. As a result, the coil L becomes ON, while the coil L,remains OFF, so that the maintaining valve 59 is actuated, however, thestop valve of the hydraulic pressure modulator is not yet closed at thebeginning of the control, since it is so constructed that is opened andclosed by the displacement of the diaphragm, therefore the hydraulicpressure still continues to increase as the driver continues to depressthe brake pedal. When the time i is reached, V gets lower than o'V,, sothat this time C, undergoes a level shift to zero level while C and Cremain at zero levels. As a result, the coil L, is switched ON and thecoil L is switched OFF actuating the valve 42 and releasing the valve 59to admit air into the chamber A so as to move the diaphragm 52 to theleft. Consequently, the stop valve is closed, and the hydraulic pressureis then decreased. During the course of such a pressure decrease, thespeed of the wheel rotation tends to recover, and V,,, gets higher than(TV, at the time t, when C remains at zero level, C is switched to level1, and C remains at zero level. As a result, the coil L, is switched OFFand the coil L, is switched ON, so that the hydraulic pressure ismaintained at a value of that time. The speed of wheel rotation, on theother hand, continues to increase, with V,,, becomming equal to V, atthe time 5. At this instant, however, V, is still much lower than Vrepresenting the actual vehicle speed, so it continues to increase,charging the capacitor. Therefore, C, and C under a level shift tolevel, while C, remains at level I, and the coils L, and L remainrespectively OFF and ON. Thus, the hydraulic pressure continues toremain constant.

At the time t, the speed of wheel rotation ceases to increase. (If thehydraulic pressure in zero, the speed of wheel rotation would reach theactual speed of the vehicle. However, as the hydraulic pressure ismaintained at a value, the speed of the wheel rotation ceases toincrease to remain at a value slightly lower than the actual vehiclespeed resulting from a slip caused by the maintained pressure.) Thecharged capacitor now begins to discharge, so that C undergoes a levelshift to zero level with C and C remaining at level 1. As a result, thecoil L is switched OFF while the coil L, is maintained OFF to render thechamber A into communication with the negative pressure, whereupon thehydraulic pressure begins to increase again. At the time t, when V,.,gets lower than V,, C, undergoes a level shift to zero level with Cremaining at level 1 and C, at zero level so as to switch the coil L ONwhile maintaining the coil L OFF so as to maintain the hydraulicpressure, and the speed of wheel rotation continues to decrease. At thetime t, when V gets lower than 0V,, C undergoes a level shift to zerolevel with C, and C remaining at zero level so as to switch the coil L,to be ON and the coil L, to be OFF. This is the same state at the timet,,, and the hydraulic pressure decreases. The states at t,,, t,,,, tand t are respectively the same as the corresponding previous states att t and t and similar cycles of operations are repeated until thevehicle stops. As will be observed from FIG. 3, there are a successionof hydraulic pressure changes, such as decreasing, maintaining,increasing, maintaining, again decreasing and so on.

Though in the forgoing embodiment, V equals V, at the peak portions ofthe curve for V,,, as shown in FIG. 3a, it is sometimes preferable toset the parameters involved so that V, is slightly lower than V,,, atpeaks of the curve for V,,, depending upon the characteristics of thevehicle. In this case, the output C, from the comparator 9 undergoes alevel shift to level I when V,. becomes higher than or equal to V, andto zero level when V gets lower than V,. The other functions areentirely the same as above.

Further, similar effects may be obtained by setting the second referencevoltage as the difference obtained by subtracting a constant voltage efrom the first reference voltage V,, that is, by maintaining therelation that the second reference voltage equals the first referencevoltage V, minus a constant voltage e.

An explanation has hereinbefore been made of a case wherein tworeference set speeds are to be predetermined, however, in order toachieve the effect of preventing the locking of the vehicle wheels bythe repeated operations of maintaining the hydraulic pressure at apredetermined value and automatically renewing the reference speed inaccordance with the invention, setting only one reference speed may beemonly reference speed is set, and FIG. shows the 5 imaginery vehiclespeed represented by V, the speed of wheel rotation represented by V thereference speed represented by o-V,, the current flowing through thesolenoid coil L, for increasing and decreasing the pressure, the currentflowing through the solenoid coil L,

for maintaining the pressure, and the hydraulic breaking pressure P inthe embodiment shown in FIG. 4.

Referring to FIG. 4, numeral 1 designates a genera- 1 these circuits,numerals 21 and 31 designate diodes for respectively blocking a reversecurrent, numeral 22 a resistor for determining the time constant of thedischarge of a capacitor 23, numerals 24 and 25 resistors for setting ofthe reference speed with the respective resistance values R and R beingsufficiently high as compared with the resistance value R of theresistor 22, numeral 32 a resistor with a resistance value R fordetermining the time constant of the discharge of a capacitor 33,numeral 34 a resistor for detecting the charging and discharging of thecapacitor 33 having a resistance value R sufficiently low as comparedwith R numerals 26 and 36 comparators, numerals l6 and 17 amplifiers toamplify the outputs of the respective comparators 26 and 36, and

numerals 18 and 19 the same solenoids as already described.

This circuit diagram has been drawn assuming that 40 all the constituentelements of the circuit have ideal electrical characteristics. In theconstruction of an actual circuit, however, a common practice is toinsert an impedance transforming device before the resistor 24 asexplained in reference to FIG. 1 in order that the resistance values Rand R may assume proper working values in accordance with thecharacteristics of the charging and discharging circuits, the voltagedividing circuit and the comparators. In FIG. 4, such an impedancetransforming device is intentionally omitted for the purpose ofsimplifying the illustration.

In this arrangement, the time constants T and T: for

discharging the capacitors 23 and 33 are respectively given by:

whereacross the voltage V, is applied, a voltage crV, is applied to thecomparator 26. The factor a is given by:

It may be set to a desired value by suitably selecting the values of Rand R That is, the reference set speed is thus represented by voltagecrV The comparator 26 also receives an input voltage V,, representingthe speed of wheel rotation, and it compares V,,, with 0V, to generatean ON signal when V,,, is lower than 0V, so as to energize the solenoid18 through the amplifier 16, thereby decreasing the braking pressure onthe wheels.

While, in the pressure maintaining signal generating 5 circuit 30, thecapacitor 33 is similarly charged by the voltage V,,, which representsthe speed of wheel rotation through the resistor 34 which more or lesscauses a voltage drop therein. During charging the capacitor 33, acharging current i flows through the resistor 34 in the direction of thearrow, so that the opposite ends of the resistor 34 are at differentpotentials. When the wheels are decelerated the current flowing throughthe resistor 34 is reversed due to the discharging of the capacitor 33.The comparator 36 discriminates whether the capacitor 33 is beingcharged or discharged, and it generates an ON signal while the capacitor33 is charged (namely, during the accelera tion of the wheels), while itgenerates an OFF signal while the capacitor 33 is discharged (namely,during the deceleration). While the speed of wheel rotation isincreasing, the comparator 36 energizes the solenoid 19 through theamplifier 17 to maintain the wheel braking pressure at a value of thattime.

The sequence of events is now described with reference to FIG. 5, wherethe abscissa represents time t and the ordinate represents the wheelrotation speed voltage V.,,, the imaginery vehicle speed voltage V, thereference speed voltage V the ON-OFF performances of the coil L, of thesolenoid 18 operating the pressure increasing and decreasing valve 42,those of the coil L, of the solenoid 19 operating the pressuremaintaining valve 59, and that of the hydraulic wheel braking pres sureP. When braking is started at the time t, to decrease V,,, the referencespeed voltage 0V, is also decreased at a constant decelerationdetermined by the time constant T, of the discharge of the capacitor 23.At the time t V gets lower than 0V whereupon the comparator 26 generatesan ON signal to energize the solenoid 18 of the pressure increasing anddecreasing valve 42, thereby decreasing the hydraulic braking pressureP. The hydraulic braking pressure P further continues to decrease untilthe time 1 is reached, when the wheel rotation speed turns to increaseto cause the comparator 36 to generate an ON signal for energizing thesolenoid 19 of the pressure maintaining valve 59, thereby maintainingthe hydraulic brak ng P essure P at a value of the time t While, inspite of the increase of the wheel rotation speed, the capacitor 23continues to discharge so long as V,, is lower than V1, and thereference speed voltage continues to decrease until V, restores tobecome equal to V, at the time t With a further increase of V,,, thecapacitor 23 is switched to be charged, and simultaneously the referencespeed voltage a-V, increases. At a little earlier time t when V getsequal to 0V the solenoid 18 is switched to become OFF. While the wheelsare increasing their speed of rotation, the hydraulic braking pressure Pis maintained at a constant value, so the wheel speed increase stops atthe time t, to again turn to decrease, switching the capacitor 23 tostart discharging again. At this instant t the reference speed voltagealso becomes maximum and turns to decrease again at a constantdeceleration. Since V is higher than 0V; and the capacitor 33 isdischarging at this time, both solenoids l8 and 19 are OFF, so thehydraulic braking pressure P begins to increase again. The foregoingsuccessive operations are repeated until the vehicle gradually comes toa stop.

It should, of course, be understood that the factor 0' may be madeunity.

I claim:

1. A control system for preventing the skidding of vehicles providedwith a hydraulic brake system including a master cylinder and wheelcylinders operated by a braking hydraulic pressure produced by saidmaster cylinder, said control system comprising:

a braking hydraulic pressure modulator having a first position forsupplying the braking hydraulic pressure produced by said mastercylinder to said wheel cylinders, a second position for stopping thesupply of the hydraulic pressure to said wheel cylinders, a thirdposition for reducing the hydraulic pressure in said wheel cylinderswhile the supply of the hydraulic pressure thereto is stopped and afourth position for restoring said reduced hydraulic pressure:

A wheel speed signal generator for generating a signal corresponding toa wheel speed;

means responsive to said wheel speed signal to generate, when a wheeldeceleration exceeds a predetermined deceleration, a first set referencewheel speed signal which decreases with a rate of reductioncorresponding to said predetermined deceleration starting from a levelof the wheel speed signal equal to or lower than that at the time whenthe wheel deceleration exceeds said predetermined deceleration and asecond set reference wheel speed signal which is lower than said firstset reference wheel speed signal,

said means including means for renewing said first and second setreference wheel speed signals when the wheel speed signal exceeds saidfirst set reference wheel speed signal and changes from an increasingstate to a decreasing state; and

braking hydraulic pressure modulator actuating means responsive to thewheel speed signal for actuating said braking hydraulic pressuremodulator a. to said third position for reducing the hydraulic pressurewhen the wheel speed signal is lower than said second set referencewheel speed signal,

. to said second position for stopping the hydraulic pressure supply tomaintain the hydraulic pressure level in said wheel cylinders at thetime when the wheel speed signal is higher than said second setreference wheel speed signal but lower than said first set referencewheel speed signal or when the wheel speed signal exceeds said first setreference wheel speed signal and is still increasing, and

e. to said fourth position for restoring the reduced hydraulic pressureor said first position for supplying the hydraulic pressure to increasethe hydraulic pressure in said wheel cylinders when the wheel speedsignal is higher then said first set reference wheel speed signal and isdecreasing.

2. A control system according to claim 1 for use with a said brakesystem which includes a path through which the braking hydraulicpressure produced by said master cylinder is supplied to the wheelcylinders and wherein said braking hydraulic pressure modulatorcomprises a stop valve disposed in said path, a fluid chamber located insaid path on the side of the wheel cylinders with respect to said stopvalve, a pressure control rod moving in and out of said fluid chamber, adiaphragm connected with said rod, two air chambers separated by saiddiaphragm, a conduit for providing communication between said two airchambers, an electromagnetic valve for stopping, when energized,communication between said conduit and one of said air chamberspositioned on the side of said pressure control rod, an electromagneticchange-over valve for switching,

when energized, said conduit to be cut off from communication with theother one of said air chambers and to communicate with atmosphericpressure,

means for bringing said other air chamber into communication withnegative pressure,

and a spring for urging said diaphragm to move sai pressure control rodinto said fluid chamber to open said stop valve by a pushing force ofsaid spring when said two electromagnetic valves are deenergized,whereby said two electromagnetic valves are selectively energized tocontrol air pressures in said two air chambers so that the movement ofsaid pressure control rod is controlled to selectively close said stopvalve and to reduce or to restore the braking hydraulic pressure in saidpath through which the braking hydraulic pressure is supplied to saidwheel cylinders.

3. A control system for preventing the skidding of vehicles providedwith a hydraulic brake system including a master cylinder and wheelcylinders operated by a braking hydraulic pressure produced by saidmaster cylinder, said control system comprising:

a braking hydraulic pressure modulator capable on actuation of assuminga first position for supplying the braking hydraulic pressure producedby said master cylinder to said wheel cylinders, a second position forstopping the supply of the hydraulic pressure of said wheel cylinders, athird position for reducing the hydraulic pressure in said wheelcylinders while the supply of the hydraulic pressure thereto is stoppedand a fourth position for restoring the reduced hydraulic pressure;

a wheel speed signal generator for generating a DC voltage correspondingto a wheel speed;

set reference wheel speed signal generating means comprising acapacitor, means for charging said capacitor with a voltage of the wheelspeed signal,

means for discharging said capacitor at a predetermined rate ofdischarge and means for dividing a terminal voltage across saidcapacitor,

said terminal voltage across said capacitor forming a first setreference wheel speed signal and an output voltage of said dividingmeans forming a second set reference wheel speed signal; and

braking hydraulic pressure modulator actuating means comprising a firstcomparator for comparing the wheel speed signal with said first setreference wheel speed signal,

a second comparator for comparing the wheel speed signal with saidsecond set reference wheel speed signal and a third comparator fordetecting whether or not said capacitor is being charged,

said actuating means being operable in accordance with the outputs ofsaid three comparators to actuate said braking hydraulic pressuremodulator a. to said third position for reducing the hydraulic pressurewhen the wheel speed signal is lower than said second set referencewheel speed signal,

b. to said second position for stopping the hydraulic pressure supply tomaintain the hydraulic pressure level in said wheel cylinders at thetime when the wheel speed signal is higher than said second setreference wheel speed signal but lower than said first set referencewheel speed signal and said capacitor is being charged, and

c. to said fourth position for restoring the reduced hydraulic pressureor said first position for supplying the hydraulic pressure to increasethe hydraulic pressure in said wheel cylinders when the wheel speedsignal is higher than said first set reference wheel speed signal andsaid capacitor is not being charged,

wherein said capacitor discharging means comprises a constant currentdischarging circuit and wherein said constant current dischargingcircuit comprises a parallel connection of said capacitor and aresistor, one end of said parallel connection being negatively biased.

4. A control system according to claim 3, wherein said wheel speedsignal generator generates a signal corresponding to a rotational speedof a propeller shaft for driving the wheels.

5. A control system according to claim 3, wherein said wheel speedsignal generator comprises means for detecting a voltage having afrequency proportional to the wheel rotation speed and afrequency-voltage converter for converting said voltage into DC voltage.

6. A control system according to claim 3, wherein said capacitorcharging means comprises a diode.

UNETED STATES PATENT @FFHIE Dated December 12, 1972 Patent No.

Takeshi Ochiai Inventor(s) It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

In the heading:

[73] Assignee: "Toyota Jidosna Kabushiki Kaisha" should read ToyotaJidosha Kogyo Kabushiki Kaisha Signed and sealed this 15th day ofJanuary 197A.

( SEAL) Attest:

EDWARD M. FLETCHER,JR I RENE D. TEGTMEYER Attesting Officer ActingCommissioner of Patents

1. A control system for preventing the skidding of vehicles providedwith a hydraulic brake system including a master cylinder and wheelcylinders operated by a braking hydraulic pressure produced by saidmaster cylinder, said control system comprising: a braking hydraulicpressure modulator having a first position for supplying the brakinghydraulic pressure produced by said master cylinder to said wheelcylinders, a second position for stopping the supply of the hydraulicpressure to said wheel cylinders, a third position for reducing thehydraulic pressure in said wheel cylinders while the supply of thehydraulic pressure thereto is stopped and a fourth position forrestoring said reduced hydraulic pressure: A wheel speed signalgenerator for generating a signal corresponding to a wheel speed; meansresponsive to said wheel speed signal to generate, when a wheeldeceleration exceeds a predetermined deceleration, a first set referencewheel speed signal which decreases with a rate of reduCtioncorresponding to said predetermined deceleration starting from a levelof the wheel speed signal equal to or lower than that at the time whenthe wheel deceleration exceeds said predetermined deceleration and asecond set reference wheel speed signal which is lower than said firstset reference wheel speed signal, said means including means forrenewing said first and second set reference wheel speed signals whenthe wheel speed signal exceeds said first set reference wheel speedsignal and changes from an increasing state to a decreasing state; andbraking hydraulic pressure modulator actuating means responsive to thewheel speed signal for actuating said braking hydraulic pressuremodulator a. to said third position for reducing the hydraulic pressurewhen the wheel speed signal is lower than said second set referencewheel speed signal, b. to said second position for stopping thehydraulic pressure supply to maintain the hydraulic pressure level insaid wheel cylinders at the time when the wheel speed signal is higherthan said second set reference wheel speed signal but lower than saidfirst set reference wheel speed signal or when the wheel speed signalexceeds said first set reference wheel speed signal and is stillincreasing, and c. to said fourth position for restoring the reducedhydraulic pressure or said first position for supplying the hydraulicpressure to increase the hydraulic pressure in said wheel cylinders whenthe wheel speed signal is higher then said first set reference wheelspeed signal and is decreasing.
 2. A control system according to claim 1for use with a said brake system which includes a path through which thebraking hydraulic pressure produced by said master cylinder is suppliedto the wheel cylinders and wherein said braking hydraulic pressuremodulator comprises a stop valve disposed in said path, a fluid chamberlocated in said path on the side of the wheel cylinders with respect tosaid stop valve, a pressure control rod moving in and out of said fluidchamber, a diaphragm connected with said rod, two air chambers separatedby said diaphragm, a conduit for providing communication between saidtwo air chambers, an electromagnetic valve for stopping, when energized,communication between said conduit and one of said air chamberspositioned on the side of said pressure control rod, an electromagneticchange-over valve for switching, when energized, said conduit to be cutoff from communication with the other one of said air chambers and tocommunicate with atmospheric pressure, means for bringing said other airchamber into communication with negative pressure, and a spring forurging said diaphragm to move said pressure control rod into said fluidchamber to open said stop valve by a pushing force of said spring whensaid two electromagnetic valves are deenergized, whereby said twoelectromagnetic valves are selectively energized to control airpressures in said two air chambers so that the movement of said pressurecontrol rod is controlled to selectively close said stop valve and toreduce or to restore the braking hydraulic pressure in said path throughwhich the braking hydraulic pressure is supplied to said wheelcylinders.
 3. A control system for preventing the skidding of vehiclesprovided with a hydraulic brake system including a master cylinder andwheel cylinders operated by a braking hydraulic pressure produced bysaid master cylinder, said control system comprising: a brakinghydraulic pressure modulator capable on actuation of assuming a firstposition for supplying the braking hydraulic pressure produced by saidmaster cylinder to said wheel cylinders, a second position for stoppingthe supply of the hydraulic pressure of said wheel cylinders, a thirdposition for reducing the hydraulic pressure in said wheel cylinderswhile the supply of the hydraulic pressure thereto is stopped and afourth position for restoring the reduced hydraulic pressure; a wheelspeed signal generator for generating a DC voltage corresponding to awheel speed; set reference wheel speed signal generating meanscomprising a capacitor, means for charging said capacitor with a voltageof the wheel speed signal, means for discharging said capacitor at apredetermined rate of discharge and means for dividing a terminalvoltage across said capacitor, said terminal voltage across saidcapacitor forming a first set reference wheel speed signal and an outputvoltage of said dividing means forming a second set reference wheelspeed signal; and braking hydraulic pressure modulator actuating meanscomprising a first comparator for comparing the wheel speed signal withsaid first set reference wheel speed signal, a second comparator forcomparing the wheel speed signal with said second set reference wheelspeed signal and a third comparator for detecting whether or not saidcapacitor is being charged, said actuating means being operable inaccordance with the outputs of said three comparators to actuate saidbraking hydraulic pressure modulator a. to said third position forreducing the hydraulic pressure when the wheel speed signal is lowerthan said second set reference wheel speed signal, b. to said secondposition for stopping the hydraulic pressure supply to maintain thehydraulic pressure level in said wheel cylinders at the time when thewheel speed signal is higher than said second set reference wheel speedsignal but lower than said first set reference wheel speed signal andsaid capacitor is being charged, and c. to said fourth position forrestoring the reduced hydraulic pressure or said first position forsupplying the hydraulic pressure to increase the hydraulic pressure insaid wheel cylinders when the wheel speed signal is higher than saidfirst set reference wheel speed signal and said capacitor is not beingcharged, wherein said capacitor discharging means comprises a constantcurrent discharging circuit and wherein said constant currentdischarging circuit comprises a parallel connection of said capacitorand a resistor, one end of said parallel connection being negativelybiased.
 4. A control system according to claim 3, wherein said wheelspeed signal generator generates a signal corresponding to a rotationalspeed of a propeller shaft for driving the wheels.
 5. A control systemaccording to claim 3, wherein said wheel speed signal generatorcomprises means for detecting a voltage having a frequency proportionalto the wheel rotation speed and a frequency-voltage converter forconverting said voltage into DC voltage.
 6. A control system accordingto claim 3, wherein said capacitor charging means comprises a diode.